The present invention relates to hydraulic suspension systems and in particular to vehicle oleopneumatic suspension systems employing sliding pillar struts.
Large load hauling trucks typically use oleopneumatic (gas over oil) sliding pillar suspension struts to provide a simple combined spring and damper suspension. These sliding pillar struts typically also provide the only mechanical connection between the unsprung mass and the vehicle and therefore are subject to loads in all directions. In particular, due to the geometry between the wheel, its connection to the strut and the strut""s connection to the vehicle, the strut bearings are subject to significant side loads.
To support side loads transmitted between the unsprung mass and the vehicle, plain bearings are generally provided within the sliding pillar struts. Plain bearings typically used do not have a low enough co-efficient of friction to obtain optimum suspension performance (at worst this may cause a strut to lock, leaving only the tyre to provide suspension). Poor ride, high tyre forces and poor handling result. Furthermore, significant forces and vibrations are transmitted from the unsprung mass to the vehicle. This can be detrimental to both the life of the truck itself and to the comfort and/or health of the person operating the truck.
Conventional suspension systems used in a wide range of applications including large load hauling trucks, have the disadvantage that they provide optimum ride and handling characteristics for only a narrow range of operating conditions. For instance, with conventional oleopneumatic truck suspensions, the struts are significantly extended when the truck is unladen and are significantly contracted when the truck is laden. This reduces the effective amount of strut travel available in both the laden and unladen states, resulting in poor utilization of the total strut travel available. Furthermore, because very little suspension travel is left in the unladen state before the struts are fully extended (and very little suspension
travel is left in the laden state before the struts are fully contracted), a relatively stiff spring rate is required which further degrades suspension performance.
It is therefore an object of this invention to overcome at least some of the aforesaid disadvantages or to at least offer the public a useful choice.
According to a first aspect of the invention there is provided an oleopneumatic strut comprising:
a cylinder having a substantially closed end and an inner bore;
a piston rod slideable within said inner bore having a proximal end which terminates within said inner bore and a distal end which extends from said inner bore;
an oil seal between said inner bore and said piston rod which thereby seals a strut volume such that axial movement of said piston rod towards said closed end of said inner bore reduces said strut volume and axial movement of said piston rod away from said closed end of said inner bore increases said strut volume; and
a hydrostatic bearing, located within said strut volume between said piston rod and said inner bore, mounted to either said piston rod or said inner bore, and venting into said strut volume;
characterised in that, in use where hydraulic fluid is supplied to said bearing, control of flow of hydraulic fluid between said strut volume and a reservoir provides control of the axial position of said piston rod relative to its corresponding inner bore.
Preferably said hydrostatic bearing is mounted to said piston rod.
According to a second aspect of the present invention, there is provided a suspension system for a vehicle comprising a plurality of suspension struts, each mounted between a vehicle body and a wheel assembly, each comprising:
a cylinder having a closed end and an inner bore;
a piston rod slideable within said inner bore having a proximal end which terminates within said inner bore and a distal end which extends from said inner bore;
an oil seal between said inner bore and said piston rod which thereby seals a strut volume such that axial movement of said piston rod towards said closed end of said inner bore reduces said strut volume and axial movement of said piston rod away from said closed end of said inner bore increases said strut volume;
a compressible mass of gas and hydraulic fluid contained within said strut volume;
a hydrostatic bearing, located within said strut volume between said piston rod and said inner bore, mounted to either said piston rod or said inner bore, and venting into said strut volume; and
a hydraulic circuit and associated control system adapted to control flow of said hydraulic fluid between said strut volume and a reservoir, thereby providing control of at least the time-averaged axial position of said piston rod relative to its corresponding inner bore.
The control system also prevents the unwanted extension of each strut (time-averaged axial extension of each piston rod relative to its corresponding inner bore) which would otherwise occur due to the flow of fluid from the hydrostatic bearing into its strut volume.
Preferably, each said strut further comprises a second hydrostatic bearing axially displaced from the first said hydrostatic bearing in a direction away from said proximal end of said piston rod and mounted between said piston rod and said inner bore.
The ability of the control system to control the ride-height of the vehicle and in particular, the ability of the control system to provide the same or similar at-rest strut displacement for the truck in both its laden and unladen states provides a number of advantages. For instance, in the embodiments described below, it allows a softer spring rate to be used thereby allowing greater suspension travel from the at rest position in both the laden and unladen states.
Preferably, said suspension system further comprises control of roll angle.
Specific embodiments of the invention will now be described with reference to and as illustrated in the accompanying Figures. These embodiments are illustrative and are not meant to be restrictive of the scope of the invention.